The app label needs to be the same that is used in the AUTH_USER_MODEL
setting to specify the custom user. For Oscar's AbstractUser, we
shouldn't set an app label; Django's autodetection should work in most
cases.

It in facts works for the test suite's custom user as well; but as we
specify app labels everywhere, we might as well specify it there.

I messed up when moving Alex Moon's code to oscar.core.compat and used
the wrong User model, defeating the point of the exercise.

This is fixed now, and the filter is also used for the UserForm that
gets used in the account section.

Fixes #1283.
Fixes #1282.

(cherry picked from commit 20df512eda)

Conflicts:
	oscar/apps/customer/forms.py

I messed up when moving Alex Moon's code to oscar.core.compat and used
the wrong User model, defeating the point of the exercise.

This is fixed now, and the filter is also used for the UserForm that
gets used in the account section.

Fixes #1283.
Fixes #1282.

Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser.  This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.

This does require a change to the way tests authenticate as we don't
actually use the username field.

* Emailbackend: Updated to use REQUIRED_FIELDS
* Demo and sandbox site now use compat.AUTH_USER_MODEL as well
* Added example custom user that gets tested for Django >= 1.5